In industrial applications of electric motors, it is frequently important to precisely control the speed or torque output of the motor. Until recently, DC motors hace been almost exclusively used in these applications because their flux and torque may be easily controlled by controlling the field and armature currents of such a motor. DC motors, however, have limitations imposed by their commutators and brushes, including the need for periodic maintenance, limitations on maximum speed in order to achieve reasonable brush life, contamination of the brushes in hostile environments, and the limited capacity of the commutator and brushes to conduct current and to withstand high-voltage operation.
Induction types of AC motors have no brushes or commutators. Induction motors are simpler, more rugged, and generally superior mechanically to DC motors. Additionally, induction motors are much less expensive than DC motors, and an induction motor can cost as little as one fifth as much as the equivalent horsepower DC motor. Until recently, however, the use of induction motors in high performance applications has been limited due to the inability of induction motor controllers to achieve torque and speed control which is as accurate as that which can be achieved with DC motor controllers. Most commercially available induction motor controllers are variable voltage source drives that operate in an open-loop, constant volts/Hz manner. While these types of controllers may be easily used with motors of different design and horsepoower, the dynamic performance of such controllers is poor, and attempts to achieve rapid torque response typically result in tripping the over-current protection of the motor or controller.
Recently, flux-feed-forward types of induction motor controllers, also known as vector-type controllers or field-oriented controllers, have been developed which are capable of producing excellent dynamic and static torque response at all speeds from a standard induction motor. To achieve accurate torque and speed response from a motor controlled by a flux-feed-forward type of controller, it is necessary for the controller to know the values of key motor parameters very accurately, especially the magnetizing inductions and rotor resistance. The value of the magnetizing inductance is required to set the flux level in the motor, and the value of the motor resistance must be known for the slip frequency to be optimally controlled.
In actual practice, it is difficult to precisely know these values. For example, a 50.degree. C. rise in temperature may increase the motor rotor resistance by 20-30%, and this change in rotor resistance will generally occur as a motor is used and heats up. The motor inductance is relatively stable with time and temperature changes. Motor inductances of different types or horsepower capacity motors, however, tend to vary widely. Thus, a flux-feed-forward motor controller designed to work with a particular motor will not provide accurate control of torque and speed when used with a different motor, even one having the same horse power. Even with nominally identical motors, small variations in the air gap and other parameters which are within mormal production tolerances may result in significant changes in motor inductance. Thus, flux-feed-forward types of controllers for induction motors have generally required that they be individually ajusted to provide optimum control over the motor output torque. This prevents motor controllers from being easily interchangeable between different motors, and, as discussed above, results in degraded control of the motor torque output as the rotor resistance of the motor changes with changes in temperature.